Genèse 34:13

But the sons of Jakob answered Shekem. and Hamor his father with subtilty, and so spake, because he had polluted Dinah their sister,

AND THE SONS OF JACOB ANSWERED SHECHEM AND HAMOR HIS FATHER WITH SUBTLETY. Now Hamor and Shechem spoke to her father and her brothers, but the patriarch did not answer them at all as his sons spoke in his place on this matter out of respect for him for since the affair was a source of shame to them, they did not want him to speak about it at all.
There is a question which may be raised here. It would appear that they answered with the concurrence of her father and his advice for they were in his presence, and it was he who understood the answer which they spoke with subtlety, and, if so, why was he angry afterwards? Moreover, it is inconceivable that Jacob would have consented to give his daughter in marriage to a Canaanite who had defiled her. Now surely all the brothers gave that answer with subtlety, while Simeon and Levi alone executed the deed, and the father cursed only their wrath. [But if all the brothers shared responsibility for the answer and the plan, why did Jacob single out only Simeon and Levi for chastisement?] The answer is that the craftiness lay in their saying that every male of theirs be circumcised, as they thought that the people of the city will not consent to it. Even if perchance they will listen to their prince and they will all become circumcised, they will come on the third day, when they were in pain, and will take their daughter from the house of Shechem. Now this was the advice of all the brothers and with the permission of their father, but Simeon and Levi wanted to take revenge of them and so they killed all the men of the city.
It is possible that Jacob’s anger in cursing their wrath was because they killed the men of the city who had committed no sin against him; they should have killed Shechem alone. It is this which Scripture says, And the sons of Jacob answered Shechem and Hamor his father with subtlety, and spoke, because he had defiled Dinah their sister, for they all agreed to speak to him craftily because of the base deed which he had done to them.
Now many people ask: “But how did the righteous sons of Jacob commit this deed, spilling innocent blood?” The Rabbi (Moshe ben Maimon) answered in his Book of Judges, saying that “sons of Noah”, Note 222. are commanded concerning Laws, and thus they are required to appoint judges in each and every district to give judgment concerning their six commandments which are obligatory upon all mankind. “And a Noachide who transgresses one of them is …

במרמה, they demanded that the people circumcise themselves expecting them to refuse. Alternatively, they thought that Shechem and Chamor, although prepared themselves to circumcise themselves, would not be able to convince the townsfolk to follow their example.

וידברו, אשר טמא; they said that the kind of ipso facto voluntary circumcision which Shechem and his father were willing to perform on themselves was not relevant after Shechem had already defiled their sister. They considered this as in the category of אתנן זונה, offering the price paid to a whore as a sacrifice on G’d’s altar.

ויענו בני יעקב, Yaakov himself did not utter a single word of falsehood or deceit, even though seeing that he had been the injured party he could have excused such conduct. He left the matter in his sons’ care and if they used subterfuge he did not interfere. The brothers did not consider their conduct as reprehensible for people who had defiled their sister and were still holding her captive.